gitian: Add -O2 for win32 build

In the win32 gitian descriptor the C*FLAGS are being overridden on
`configure`, which causes the built-in optimization flags to be removed.
Add `-O2` manually (but not `-g` as we're not doing anything with the
debug information).

Improves performance of win32 gitian builds.

As a bonus, fixes issue #3497.

contrib/gitian-descriptors/gitian-win32.yml

@@ -30,6 +30,7 @@ script: |
   #
   STAGING=$HOME/staging
   HOST=i686-w64-mingw32
+  OPTFLAGS='-O2'
   #
   mkdir -p $STAGING
   cd $STAGING
@@ -43,12 +44,12 @@ script: |
   export PATH=$STAGING/host/bin:$PATH
   export TAR_OPTIONS=--mtime=`echo $REFERENCE_DATETIME | awk '{ print $1 }'`
   ./autogen.sh
-  ./configure --bindir=$OUTDIR --prefix=$STAGING --host=$HOST --with-qt-plugindir=$STAGING/plugins  --with-qt-incdir=$STAGING/include --with-qt-bindir=$STAGING/host/bin --with-boost=$STAGING --disable-maintainer-mode --with-protoc-bindir=$STAGING/host/bin --disable-dependency-tracking CPPFLAGS="-I$STAGING/include" LDFLAGS="-L$STAGING/lib" CXXFLAGS="-frandom-seed=bitcoin"
+  ./configure --bindir=$OUTDIR --prefix=$STAGING --host=$HOST --with-qt-plugindir=$STAGING/plugins  --with-qt-incdir=$STAGING/include --with-qt-bindir=$STAGING/host/bin --with-boost=$STAGING --disable-maintainer-mode --with-protoc-bindir=$STAGING/host/bin --disable-dependency-tracking CPPFLAGS="-I$STAGING/include ${OPTFLAGS}" LDFLAGS="-L$STAGING/lib ${OPTFLAGS}" CXXFLAGS="-frandom-seed=bitcoin ${OPTFLAGS}"
   make dist
   mkdir -p distsrc
   cd distsrc
   tar --strip-components=1 -xf ../bitcoin-*.tar.*
-  ./configure --bindir=$OUTDIR --prefix=$STAGING --host=i686-w64-mingw32 --with-qt-plugindir=$STAGING/plugins  --with-qt-incdir=$STAGING/include --with-qt-bindir=$STAGING/host/bin --with-boost=$STAGING --disable-maintainer-mode --with-protoc-bindir=$STAGING/host/bin --disable-dependency-tracking CPPFLAGS="-I$STAGING/include" LDFLAGS="-L$STAGING/lib" CXXFLAGS="-frandom-seed=bitcoin"
+  ./configure --bindir=$OUTDIR --prefix=$STAGING --host=i686-w64-mingw32 --with-qt-plugindir=$STAGING/plugins  --with-qt-incdir=$STAGING/include --with-qt-bindir=$STAGING/host/bin --with-boost=$STAGING --disable-maintainer-mode --with-protoc-bindir=$STAGING/host/bin --disable-dependency-tracking CPPFLAGS="-I$STAGING/include ${OPTFLAGS}" LDFLAGS="-L$STAGING/lib ${OPTFLAGS}" CXXFLAGS="-frandom-seed=bitcoin ${OPTFLAGS}"
   export LD_PRELOAD=/usr/lib/faketime/libfaketime.so.1
   export FAKETIME=$REFERENCE_DATETIME
   export TZ=UTC